如何避免java.lang.StackOverflowError？

Question

I implemented a flood fill algorithm to my paint application. There were no problems for my code on that algorithm.

When I test the program, I noticed that the flood fill works fine for small enclosed areas but when the flood fill applied to large areas, I got java.lang.StackOverflowError and the large area was half filled after repainting. I know that Java have limited call stack for recursive methods, I'm not sure how can I optimize my code to cope with this problem, should resizing my bufferedimage necessary?

Code:

import java.awt.*;

import java.awt.event.*;

import java.awt.image.BufferedImage;

import javax.swing.*;

public class MinimumVerifiableExample extends JFrame {
    private static final long serialVersionUID = 1L;

    private final int WIDTH = 800;
    private final int HEIGHT = 600;

    private PaintPanel panel;
    private JButton button;

    private MinimumVerifiableExample() {
        super("Paint App Plus");

        panel = new PaintPanel();
        button = new JButton("Fill with mouse click");

        button.addActionListener(e -> {
            panel.setFloodFill(Color.RED);
        });

        setSize(WIDTH, HEIGHT);
        setLocationRelativeTo(null);

        setLayout(new BorderLayout());

        add(panel, BorderLayout.CENTER);
        add(button, BorderLayout.SOUTH);

        setResizable(false);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(() -> {
            MinimumVerifiableExample frame = new MinimumVerifiableExample();
            frame.setVisible(true);
        });
    }

    private class PaintPanel extends JComponent implements MouseListener, MouseMotionListener {
        private static final long serialVersionUID = 1L;

        private final int canvasWidth = 784;
        private final int canvasHeight = 526;

        private BufferedImage canvas;
        private boolean floodFill;
        private Color fillColour;

        private boolean painting;
        private int prevX;
        private int prevY;
        private int curX;
        private int curY;

        private PaintPanel() {
            canvas = new BufferedImage(canvasWidth, canvasHeight, BufferedImage.TYPE_INT_RGB);
            floodFill = false;
            fillColour = null;

            painting = false;

            Graphics2D paintBrush = canvas.createGraphics();

            paintBrush.setColor(Color.WHITE);
            paintBrush.fillRect(0, 0, canvas.getWidth(), canvas.getHeight());
            paintBrush.dispose();

            addMouseListener(this);
            addMouseMotionListener(this);
        }

        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.WHITE);
            g.fillRect(0, 0, canvas.getWidth(), canvas.getHeight());
            g.drawImage(canvas, getInsets().left, getInsets().top, canvasWidth, canvasHeight, this);
        }

        public void setFloodFill(Color fillColour) {
            floodFill = true;
            this.fillColour = fillColour;
        }

        private void floodFill(int x, int y, Color target, Color previous) {
            if (x > canvas.getWidth() || x < 1 || y > canvas.getHeight() || y < 1)
                return;

            if (canvas.getRGB(x, y) != previous.getRGB())
                return;

            previous = new Color(canvas.getRGB(x, y));
            canvas.setRGB(x, y, target.getRGB());

            floodFill(x + 1, y, target, previous);
            floodFill(x, y + 1, target, previous);
            floodFill(x - 1, y, target, previous);
            floodFill(x, y - 1, target, previous);
        }

        private void updateBoard() {
            Graphics2D paintBrush = canvas.createGraphics();
            paintBrush.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
            paintBrush.setPaint(Color.BLACK);

            paintBrush.setStroke(new BasicStroke(10, BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND));
            paintBrush.drawLine(prevX, prevY, curX, curY);

            paintBrush.dispose();
        }

        public void mousePressed(MouseEvent e) {
            if (floodFill) {
                floodFill(e.getX(), e.getY(), fillColour, new Color(canvas.getRGB(e.getX(), e.getY())));
                repaint();

                floodFill = false;
                return;
            }

            if (painting) return;

            prevX = e.getX();
            prevY = e.getY();

            painting = true;
        }

        public void mouseReleased(MouseEvent e) {
            if (!painting) return;

            curX = e.getX();
            curY = e.getY();

            painting = false;
        }

        public void mouseDragged(MouseEvent e) {
            curX = e.getX();
            curY = e.getY();

            if (!painting) return;

            updateBoard();
            repaint();

            prevX = curX;
            prevY = curY;
        }

        public void mouseClicked(MouseEvent e) {}
        public void mouseEntered(MouseEvent e) {}
        public void mouseExited(MouseEvent e) {}
        public void mouseMoved(MouseEvent e) {}
    }
}

Answer 1

Solution:

    private class StackItem {
        private final int x;
        private final int y;
        private final Color previous;

        public StackItem(int x, int y, Color previous) {
            this.x = x;
            this.y = y;
            this.previous = previous;
        }
    }

    private void floodFill(final int initialX, final int initialY, final Color target, final Color previous) {
        Stack<StackItem> stack = new Stack<>();
        stack.push(new StackItem(initialX, initialY, previous));

        while (!stack.isEmpty()) {
            StackItem stackItem = stack.pop();
            if (stackItem.x > canvas.getWidth() || stackItem.x < 1 || stackItem.y > canvas.getHeight() || stackItem.y < 1)
                continue;

            if (canvas.getRGB(stackItem.x, stackItem.y) != stackItem.previous.getRGB())
                continue;

            Color previousColor = new Color(canvas.getRGB(stackItem.x, stackItem.y));
            canvas.setRGB(stackItem.x, stackItem.y, target.getRGB());

            stack.push(new StackItem(stackItem.x + 1, stackItem.y, previousColor));
            stack.push(new StackItem(stackItem.x, stackItem.y + 1, previousColor));
            stack.push(new StackItem(stackItem.x - 1, stackItem.y, previousColor));
            stack.push(new StackItem(stackItem.x, stackItem.y - 1, previousColor));

        }


    }

Please, pardon the use of continue . I wanted to keep the structure of original solution similar to this one. I recommend though to restrain from using it.

As you can see, this is a direct approach on how to translate recursion into a loop. Instead of using JVM stack, which has limited size, we are using a collection, which uses JVMs heap.

Class StackItem is simply a representation of all arguments of a recursive function. Argument target does not change, so it is not part of it. Every recursive call is a equal to pushing new argument to our Stack structure. Every invocation of a "recursive" function is equal to poping the argument from top and executing logic using this argument.

Answer 2

The simplest solution is to carefully inspect the stack trace and detect the repeating pattern of line numbers. These line numbers indicate the code being recursively called. Once you detect these lines, you must carefully inspect your code and understand why the recursion never terminates.